Heat-sensitive copying paper



Dec. 22, 1953 c. s. MILLER ET AL 2,653,654

HEAT-SENSITIVE COPYING PAPER med May 15, 1952 2 sheets-sheet 1 l @c5/#W76 copy. j j? Enz/@M5075 @MMMSLM fa m65/ M wO/wy Dec. 22, 1953 c. s. MILLER ET AL HEAT-SENSITIVE COPYING PAPER Filed May 15, 1952 Patented Dec. 22, 1953 UNiTEo stare TENT HEAT-SENSITIVE COPYING PAPER of Delaware Application May 15, 1952, Serial No. 288,003

7 claims. 1

This invention is concerned with duplicator sheet material or copying-paper useful in preparing copies of printed matter or the like, and with compositions and methods related thereto, The invention particularly contemplates heatsensitive coatings and coated sheet materials of a novel type, and methods of utilizing such materials in preparing duplicate copies of printed. or other graphic subject-matter by simple exposure to high intensity irradiation and without subsequent developing or fixing operations.

This application is a continuation-in-part of our copending application Serial No. 747,339, filed May 10, 1947.

Prior art copying-papers of which We are aware, which are subjected to exposure to visible light or similar forms of radiant energy in the preparation of copies of graphic subject-matter, must be protected from light before use, and require various processing steps, in addition to the initial controlled exposure, for the production of visible light-stable copies.

Our new heat-sensitive copying-paper, on the contrary, is not itself aiected by radiant energy such as visible or ultra-violet light, and may therefore be left exposed to such radiations both before and after printing. Nevertheless, radiant energy may be employed as the sole agency for the production of visible light-stable and permanent copies of graphic subject-matter on our novel copying-paper, or on other surfaces carrying our novel heat-sensitive compositions, by processes as hereinafter set forth.

The processes by which we are enabled to achieve this result, e. g. in the preparation of `copies of the printed pages of books or the like,

involve the irradiation of the printed surface, the resultant formation of an elevated-temperature pattern corresponding to the graphic subject-matter of such printed surface, and the utilization of this elevated-temperature pattern in the development of a facsimile copy of the graphic subject-matter on the heat-sensitive copying- 'a relatively long time at a specific temperature, "or heating over a wide range of temperature, to produce a visible change of suflicient intensity.V

'Ihe change obtained at elevated temperatures may be reversible on cooling, i. e. the product exhibits thermotropy. These and other deficiencies of prior art materials are avoided by the use of our novel compositions and products, particularly in connection with the copying of printed matter such as books, drawings or diagrams, pictures, etc. by means of our novel copying-paper herein described and illustrated. y

The heat-sensitive,compositions of our invention are comprised of solid reactants which are potentially chemically capable of irreversibly and rapidly reacting at normal room temperature to produce a visibly dierent reaction product, but which are normally physically prevented from so reacting. The structure is so designed that an increase in temperature-to a predetermined level allows the reaction to take place. The reaction is believed to be initiated by the melting, or softening, or other physical change, of one or more of the reactive substances. Whatever the specic physical change employed, the application of heat results in an immediate reaction of the reactants and formation of a colored, opaque, or otherwise visibly different reaction product.

The rapid rate of reaction obtainable by this means is particularly advantageous vwhere the reactive material in sheet form is to be used as a heat-sensitive copying-paper, For the most effective reproduction of drawings orthe like containing fine lines as well as massive dark areas, a high contrast number, as hereinafter defined, must be obtained when the sheet is heated through a narrow range of temperature not greater than about 20 C. and preferably about 5 C., within a period of time not greater than one second, and preferably within about onetenth of a second, in order to obtain desirable contrast and detail.

Where these limitations are exceeded, the reproductions obtained may be blurred, fuzzy, or otherwise of poor quality. For example, a heatsensitive sheet material which darkens over a wide range of temperature, e. g. which provides a contrast number of 0.2 when heated to C.

` but does not reach a contrast number of 0.8 until heated to C., Will ordinarily not produce a print having good contrast between light and dark areas. y

A convenient method of determining the rate rof reaction as well as the required temperature of activation for a particular copying-paper involves placing portions of the sheet against a series of metal surfaces each at a different but controlled temperature, andv for various intervals of'time.

3 Our preferred copying-papers, when so tested, show a reaction from the original state to a color intensity or opacity equivalent to a contrast number of at least about 0.4, and preferably about 0.9, when heated at the required temperature level and with-in a temperature range of not more ythan about C., .and within a contact time of not more than about one-tenth second. The reactive components in the copying-sheet are stable 4at temperatures less than about .60 VC. but are rapidly visibly inter-reacted when the copyingsheet is heated to 120 C.

In determining the contrast number of a copying-paper, suitably exposed samples having printed (darkened) and unprinted (undarkened) areas are held against a fiat magnesium carbonate block (a standard magnesium carbonate Surface as dened in the paint industry). A beam of white light (north exposure daylight) is directed against .the outer surface cf the sample at an vangle lof 45, and the intensity of the light reflected normal to the surface is ymeasured by means of a suitable photometer. The contrast vnur nber is defined as the following ratio I u Y I d I; .y where I represents intensity, and subscripts u land d refer to lm,easurements made over the un- ,darkened area and darkened area, respectively.

The following examples of :specific compositions and structures will serve more `clearly to point out and explain the novel concept here involved, vbut are lno1-to be construed Vlas limitative.

EEQAMPLE '1 Reactant A To an ,aqueous solution .of three ,mois of the sodium soap of commercial triple-pressed stearic acid of melting-point approximately V53" C., and which supposedly contains `a minor amount of other higher fatty acids., etc., yadd an aqueous solution .of one mol of ferrie sulfate. Filter the precipitated ferric stearate, wash separately with water and alcohol, ,and dry at room temperature. The solid product softens or melts within the range of 'l0-95 C. Disperse 500 grams ,of the the powder in two liters of denatured ethyl alcohol, e. Ponsolve, by ,grinding with flint Reacteur C Dissolve 1.0 grams `,of oxalic acid in 100 milliliters of ethyl alcohol.

Binder Dissolve 150 .grams .of .polyvinyl butyral in one liter of ethyl alcohol.

Composition in `liquid form: m1. Reactant A, 500 Reactant B 100 Reactant :C (optional)v .40 Binder 200 In mixing and temporarily maintaining the mixture of Reactants A and B in a volatile vehicle such as denatured ethyl alcohol, a slight discoloration may sometimes be noted. This is presumably due to solution of traces of one or both of the substantially insoluble reactants vin the liquid 'vehicle and the resultant reaction of the ,dissolved materials to produce a dark-colored reaction product. The presence of a trace of oxalic acid, which forms a complex with iron and consequently may be considered to render inaccessible any rdissolved orv previously reacted iron, discharges and/or prevents the formation of the slight discoloration thus otherwise produced. Citric acid, which likewise forms an iron complex, is also effective. In many cases the discoloration produced, even in the absence of these modifying reactants, is so slight as not to be troublesome, particularly Awhere adeguate precautions are taken in preparing and in applying the temperaturesensitive composition.

Coated transparent sheet material The above lioluidY composition is uniformly spread, as by means of a spreader bar or knive coater, over one surface Aof a thin transparent lm of plasticized regenerated cellulose (cellophane) to a thickness of approximately 3 mils, and is allowed to dry `at room temperature. The resulting visibly transparent coated sheet product rapidly .darkens when heated to or above about C., and is suitable for use as a temperaturesensitive copying-paper.

The darkening here obtained is thought to result from a preliminary melting or softening of the ferrie stearate, making possible the mixing or rio-dissolving of lthe two reactants and their resulting rapid. reaction to form a dark-colored and opaque reaction product. This may be inferred from the yfact that the darkening of the sheet is obtained on heating the copying-paper to a temperature closely approximating the melting `or softening point oi the particular ferric soap used. A similar color change may be produced at room temperature by adding a suitable solvent, e` g. benzol, to the otherwise non-reacting mixture of the two powders.

This reaction and rapid color change does not take place on exposure of the mixture, or of the coated sheet material, to radiant energy such as visible or infra-red light in the absence of the required temperature increase. Some darkening of the .coated transparent sheet material as above prepared may be observed on prolonged exposure to light, and particularly to high intensities of actinic radiation; but this is believed to be due to the effect of certain radiations in decomposing one or both of the reactants separately, and may readily be prevented by applying a suitable filter over the temperature-sensitive coating. A thin surface coating of a mixture of 3 lgrams of 4,4-dhydroxy-benzalazine and l5 grams of polyvinyl butyral, applied from solution in grams of alcohol, provides an effective filter for this purp 0S e EXAMPLE 2 To obtain a composition having improved aging characteristics and which will react when heated to a somewhat lower temperature, similarly prepared ferrie myristate, made with commercial Vmyristic acid, Yis substituted for the ferric vstearate of Example l, and a reaction product of hexa- ,methylenetetramine and .gallic Yacid l(a substituted trihydroxy benzene), is substituted `for .the pyrogallic acid product there described.

This second ingredient is prepared as follows: 280 grams of hexamethylenetetramine are dissolved in one liter of water, and 188 grams of gallic acid are dissolved in one liter of ethyl alcohol. One volume of the first solution is mixed with two volumes of the second, and the resulting precipitate is then recovered and dried. It s ground into heptane, by means of a ball mill, to form a 25 dispersion.

Ferrie myristate softens and melts at somewhat lower temperatures than the stearate. Since the myristate is somewhat soluble in alcohol, heptane is used as the liquid vehicle, which necessitates the substitution, for the alcohol-soluble but heptane-insoluble polyvinyl butyral, of a heptanesoluble binder. In a speciic example, natural rubber was used as the binder. Heptane is substantially a non-solvent for both ferrie myristate and the reaction product of hexamethylenetetramine and gallic acid. There is ordinarily no visible evidence of reaction of the two materials in freshly prepared mixtures in heptane, although reaction takes place rapidly at temperatures below that required for activation of the coated copying-paper, and usually at or even below normal room temperatures, as in the case of the reactants of Example l, in the presence of a suitable solvent such as benzene. The final composition of the fluid mass prepared according to the above description and ready for coating is approximately as follows:

Parts by weight Ferrie myristate 100 Hexamethylenetetramine-gallic acid reaction product 25 Rubber 25 Heptane 500 This composition was coated on a thin paper, i. e. an unfilled ax paper of one mil thickness, such as is used for cigarette-paper. The mixture was applied by means of a knife-coater set to give a coating orice of approximately 3 mils, and was dried at room temperature. The coated and at least partially impregnated sheet product was faintly tan in color and appeared semi-transparent. On heating to about 70 C. it rapidly darkened and became opaque. Exposure to light alone had substantially no eiect on the appearance of the unheated material. Scribing or otherwise mechanically compacting or disrupting the surface coating did not initiate the colordeveloping reaction.

For improved permanence, the rubber used as binder may be protected, as by the addition of suitable antioxidants r other age-resistors; or various synthetic polymers or resins may be substituted for natural rubber where their properties, e. g. color, permanence, etc., permit or encourage such substitution.

A particularly desirable copying-paper, capable of rapidly producing a high contrast ratio when heated within a range of only C., was produced by employing the hexamethylenetetraminegallic acid reaction product in conjunction with ferric stearate prepared from highly purified steario acid.

EXAMPLE 3 Ferrie stearate in 5 times its weight of stearic acid was emulsied in a hot concentrated solution of glue in water. To the cooled solution was added a water solution of tannic acid. The resulting well-mixed composition was coated on paper and dried at room temperature. The sheet was reddish-brown, but turned black when heated as in the preparation lof copies of graphic subjectmatter by our novel method.

EXAMPLE 4 Ferrie stearato was dispersed in acetone by ball milling as in the previous examples, and a preformed solution of ethyl cellulose was added, to a nal formula of approximately 12% ferrie stearate, 6% ethyl cellulose, and 82% acetone. The mixture was coated in a thin uniform layer on one mil condenser tissue, a thin, tough paper, and dried at room temperature. The weight of the dry coating of ferric stearato particles uniformly dispersed in ethyl cellulose was about 0.68 gram per square foot of surface.

Gallic acid, which is a solid at normal room temperature, was dissolved in an acetone solution of ethyl cellulose, to a nal formula of approximately 5% gallic acid, 9.5% ethyl cellulose, and 85.5% acetone. This solution was coated in a thin layer over the dry ferrie stearato-ethyl cellulose layer, and the sheet again dried at room temperature. The dry weight of the second coating was about 050 gram per square foot.

The completed sheet material was useful as a copying-paper, showing high contrast and good detail in the copy. The ferrie stearato component softens or melts at 'l0-95 C., and at least the major portion of the color change in the copying-sheet occurs over substantially the same temperature range.

EXAMPLE 5 Fifty parts by weight of ferrie stearate were dispersed in 1500 parts of acetone by ball milling. Separately, parts of a phenolic resin were dispersed in 400 parts of yacetone by similar procedure. The phenolic resin was a solid heatadvancing partial reaction product of para-tertiary-butylphenol and formaldehyde, melting at 14A-158 F. (62-70" C.). Such a resin is soluble in drying oils and aromatic hydrocarbons but insoluble in alcohols and only slightly soluble in acetone.

Two parts of the ferrie stearate dispersion were mixed with one part of the resin dispersion, and the mixture was coated on thin paper and dried at room temperature. The resulting sheet material was useful as a heat-sensitive copyingpaper, but the heat-sensitive coating had a tendency to stick to contacting surfaces, particularly during the copying process. Substitution of a higher-melting heat-advancing phenolic resin for the resin hereinbefore described eliminates this tendency, resins melting above C. providing fully non-sticking surfaces with, however, some reduction in contrast in the copy obtained. Sticking is also readily avoided, and with no signicant reduction in contrast, by incorporating small amounts of a nlm-forming binder, such as ethyl cellulose, in the mixture prior to coating.

Other combinations of solid visibly inter-reactive materials which have provided eifective heat-sensitive copying-papers when coated on various paper or film backings in the form of dispersions in solutions of hn-forming binders include ferrie stearate-triethyl sulfonium tannate; ferric stearate-cadmium tannate; and ferric stearate-ammoni'um salicylate. Ferrie myristate, ferrie palmitate, and similar salts of other organic acids softening or melting at temperatures within the range of about o O-120 C. may

be substituted for the ferrie stearate'in thesev compositions. v

In, these exampleagthe visible changeobtained on activation of the coated material is the result of a combination between the iron of the ferrie stearate or equivalent and the phenolic portion of pyrogallic .o r gallic acid, tannates .salicylates or the like. A Alarge number of :other phenols which similarly ,provide .colored .reaction prod ucts with iron -salts are listed by wesp .end Brode inthe Journal of theAmericanChemical Society. volume 56, at pages 1039 10510,

The reactive compositions .of the foregoing cxamples .each comprise at least .two ksolidsensitizing components which when ,placed in sufficiently intimate contact, as for example by dissolution of one or both v`of the .components .in a 4suitable solvent, are capable .even at normal room .temperature of producing an .intense color or some other visible manifestation .of A.chentlical reaction. A @bonding agent vfor conveniently supporting and bonding the reactive .components is included; such a binder, or `at least some reactive com' ponent thereof, may itself serve as Vone of the color-producing reactants. It will be apparent, from the .several .specific `examples set forth `here.- inabove, that .a wide variety lof `cor-ilnositions having these qualifications may .be `prepared.

The reactive solid components, `eifher individually or as a pre-formed mixture, .are conveniently applied .to paper .or .other supporting structure as a dispersion in .a .solution .of .a bonding agent in a suitable volatile vehicle, as Adescribed in the above examples. The bonding agent assists in retaining the .reactants on the surface of the support. However, other vmethods .of ,apply- 'ing the reactants .to the .supporting .surface .and .of maintaining cthem in proper relationship thereon may .alternatively ,be employed. For example, .a .polymerizable monomer may .be substituted for the solution .of bonding agent; after application, themonomer may be .polymerized in situ to .form a binder lni. The powdered Vreactants ,may be .dispersed within, or `on `the .sur-- ace .o i, a fibrous web or other SuDPD'Dg structure in the substantial absence of any added bonding agent. Additionally, .the use .of a filmforming bonding agent, .such .ior yexample vas the polyvinyl butyral .of Example 1 or .the .etltiylcelluF lose vof .Example .4, as .a .self-.supporting .lrn as well as a binder .and carrier for .the reactive nlgredients is also contemplated. In this type .of

product, the .film-forming .composition .containing the coloreproducing .reactants may be coated and dried on a temporary support, .and the dry film vSubscsilently .removed by stripping .to provide an exceedingly .th-in .copying-paper. `.Compositions employing .a .reactant .or reactants which .are themselves .nlm-.forming or which 'have adequate adhesion to a supporting web, .or-

dinarily require `no .auxiliary bonding or nlmorming agent.

Certain advantageous results may be obtained by proper selection and proportioning of the bonding agent as well as the reactants, and in our ,preferred compositions we Atherefore A contemplate the use of .asuitable'finert bonding .agent or .combination of bonding agents insignificant proportions. The .degree .of contrast Y obtainable with our copying-paper is readily controlled, for example, by suitably proportioning .the relative amounts of binder ,and .of reactants. Thus', Ain.- creased contrast has been .obtained by ,increasing Vthe percentage o f binder, .and thereby presume bly increasing the distance and the .amount .of non-reactive material vbetween or around the in .dividual particles of reactants. Converseim a reduction in 'the percentage of'bonding agent reduces the .contrast .and increases the .detail obtainable in the resultant copy.

.Changes in the particle size and .shape .of .any one or `all .ofthe reactan-t materials, and in the relative amounts vof the individual reactants, will .also have Ysome `eec't on the results obtained. Sto'ichiometric vrelationships between reactants may in many Vcases be determined, but are not necessarily most advantageous. The proportions shown in the specific examples of the particular reactants therein disclosed have been found to yproduce good results, and may be used as a guide for the proper proportioning of other reactants; 'but many variations will be found to produce acceptable products and are here contemplated. Y Y

Where desired, various inert'mater'ials, such for example as pigments or the like, may beY added to the sensitizing compositions of our invention. Additional surface coatings, e. g. of nlmforming materials, may be applied as protective layers, or to impart desirable color, or for other purposes. Other modifications will be apparent.

n order to assure a .clear understanding .of the structure of our novel product and the manner in which it may utilized, e. g. in the `copying of printed matter, reference is made to the accompanying drawing, in which:

Figure 1 is a conventionalized and enlarged cross-section of one example of a suitable heatsensitive coating on a supporting member.

'Figures 2, 3 and 4 are' perspective views .(partially cut away) of three dilerent arrangements of a printed surface in relation to the coated surface of the' copying-paper in obtaining facsimile reproductions of printed or other graphic matter .on the heat-sensitive coated copyingpapers of this invention; and

Figure A5 isa diagrammatic, sectional view of one example of structure, including a suitable source of radiant energy, that may be employed for :copying printed sheet material.

In Figure '1, a heat-sensitive layer I0, consisting `of two finely-divided vreactant materials Il and l2, the individual solid Aparticles of which are contained within a binder material 13, is supported on a supporting Ymember i4.

In Figure 2, a transparent heat-sensitive copying paper It, ,consisting .of a heat-sensitive layer i0 Aon a thin transparent backing IB (both shown ih cut-away section) is placed against a printed page Il, `with the uncoated surface of the transparent'backing l in contact with the printed characters I3 of which .a facsimile copy is -desired. The transparent backing i6 of Figure 2 is .a specic member of the class more generically represented in VFigure 1 by supporting member id. On exposure of the composite to intense illumination, as herein elsewhere explained, and in this v.case with the transparent heat-sensitive sheet .t5 being between the source of light and the printed surface, a differential darkening, or other visible change, indicated by .darkened area I9, .is 'rapidly obtained inthe heat-sensitive layer ID, corresponding toY the printed character I8 therebelow.

.The nished print .obtained by the procedure outlined above in connection with Figure 2 is .directly readable from the Asurface carrying the heat-sensitive layer.V Transparency of the sheet material I5 to visible light is therefore not essential for. subsequent reading of the copy obtained. Consequently the requirement of transparency here refers to the ability of the sheet to pass radiant energy of the wavelength employed in the copying process. The use of visible light thus imposes the requirement of visible transparency, as manifested, for example, in the construction embodying cellophane and described under Example l. With other wavelengths, e. g. in the high-energy1 infra-red region, sheet material which is transparent to the infra-red but which may appear translucent or even opaque to the eye may be used and is here contemplated.

In Figure 3, the heat-sensitive layer i of the (visibly) transparent heat-sensitive copyingpaper i5 is placed directly in contact with the printed characters i8 of the printed page Il. High intensity radiation directed against the transparent backing i 6 results in a visible change, in the sensitive layer IQ, corresponding to the printed characters i8, indicated by the darkened area I (partially concealed in the drawing). The resulting copy is a direct facsimile of the printed original when viewed through the visibly transparent backing i6.

In Figure 4l, the heat-sensitive coating Ii) is disposed, as shown in cut-away section, on a supporting member I4, which maybe either transparent, translucent or opaque, but which should be at least reasonably non-conductive to heat. A thin opaque printed page 23, which may or may not carry printed characters on the under side, but which has printed characters I8, of which a copy is desired, on the outer side, is placed against the sensitive layer ill. Proper irradiation of the outer printed side of the printed page 2% results in the formation of a visible facsimile copy or darkened area i9, on the sensitive layer it, corresponding to the printed characters i8, with no undesirable blurring or ghostimage formation being obtained from any similar printed characters which may be present on the under side of the page 2i).

In Figure 5, which illustrates in cross-section one means by which reproductions of printed matter may conveniently be made according to the principles set forth in connection with Figure 2, an electrically-activated high-intensity source of useful radiation such as light rays 23, comprising an incandescent line lament 2l in transparent evacuated envelope 22, is shown supported within a movable trough-like reflector 2d of elliptical cross-section. Filament 2i is located at one focal line of the elliptical reflector; consequently light rays 23, emanating therefrom, are focused at the other focal line, which is caused to coincide with the printed surface of printed page Il, carrying printed or other graphic characters i8 of which a facsimile copy is desired.

Page il rests on the uncoated surface of a sheet of transparent heat-sensitive copying-paper i5, consisting of a transparent backing i5 and a heatsensitive coating i0. The copying-paper rests on a transparent heat-insulating sheet material 25, which may be an open-mesh screen such as silk bolting-cloth, and this screen in turn rests on a transparent support 26, which may be a glass plate, preferably of heat-resistant glass. (Where support 2S is inherently sufficiently heat-nonconductive or is suitably roughened or otherwise treated at the surface so as to presen@ nonconductive surface to the copying-paper i5., the separate thickness of insulation 25 may be eliminated.) The copy-paper i5 and page Il are held in close and heat-conductive contact by means of a heat-insulating cover 2l, which may be a Wooden cover, as shown in Figure 5, or may be constructed of any other suitable material including a soft and flexible material such as a pneumatic or hydraulic pressure-bag. If desired, the cover 21, and the underlying printed page Il, may be maintained at any desired temperature below the printing temperature of the copyingpaper l5, by suitable temperature control means. The elliptical reflector 24, with its enclosed light source, is movable across the surface of the thermographic assembly, as indicated by the double arrow 28, by suitable means (not shown), in a plane such that the light rays 23 are continuously focused at the printed surface of the printed page l'l. The rate of movement of the reflector j assembly is adjustable, to compensate for differences in the thickness and heat conductivity of the page to be copied, here shown as printed page I1, and also to compensate for differences in the sensitivity of the temperature-sensitive layer It, the intensity of light output, and various other factors.

For rapid copying of text-book pages, letters, or the like with the apparatus of Figure 5, it has been found convenient to use a 200G-watt coiled tungsten line-filament v incandescent lamp, 10 inches long, with "Pyrex glass envelope, mounted in an elliptical reflector 24. The resulting concentration of radiation is of such high intensity as to cause charring or burning if allowed to remain focused on the same section of paper for more than about one second. With this light source, and with temperature-sensitive copying-paper such as that described under the various examples, clear unfogged facsimiles may be obtained. However, many other sources of radiation, including ordinary high-intensity incandescent lamps, photoash lamps, electric arc, infra-red lamps, concentrated sunlight, etc., have been found to be useful, although commercially less effective, in various thermographic assemblies using our heat-sensitive copying-papers.

Many modifications, both in preparing the reactive compositions and the coated materials, and in utilizing the products in the direct reproduction of graphic subject-matter, have been described. While the importance of each of these and other modifications in the commercial application of the principles involved will become obvious in View of the foregoing description, it will be even more apparent that the underlying principle, on which the novel phenomena herein presented are based, remains the same in all of the applications and modications thereof. This principle involves the selection and blending of two or more normally solid reactants in such a manner that the reactants are prevented from combining or reacting with each other until the blend is heated to or above a predetermined range of temperature. When such a temperature is reached, however, the reactants are enabled rapidly to combine or react, whereupon there is produced a visible indication of such action, e. g. a color change. While the action thus obtained is of value under a wide variety of conditions, it has been shown to be particularly valuable in making available, as a new article of manufacture, a heat-sensitive copying-paper suitable for the preparation of visible light-stable copies of graphic subject-matter by methods involving only the exposure of such matter to intense irradiation while in heat-conductive relationship with the heat-sensitive material.

Having now described our invention in terms of specific examples and embodiments, but without intent to be limited thereto, what we claim is as follows:

l. A heat-sensitive copying-sheet for making direct, high contrast, clear detail copies of graphic subject-matter asherein described, saidA copying'- sheet comprising a support having low thermal conductivity, and a heat-sensitiveV layer containing, uniformly dispersed in a nlm-forming, transparent binder non-fusing at temperatures` below about 120 C., a multitude of' closely spacedl particles of at least two visibly inter-reactive solids of which one is a liquid' at temperatures abovenormal room and storage temperatures and within the range of about 60-120 C. and'v is an iron salt of a solid long-chain fatty acid, and the second inter-reactive particulate solid isl a phenol, said inter-reactive solids in said heat-sensitivelayer beingA stable at temperatures-lessv than about 60: C. and being rapid-lyl visiblyl inter-reacted when the copying sheet is heated to 120 C;

2. A heat-sensitive copying-sheet for making direct, high contrast, clear' detail copies of' graphic subject-matter as herein described, said copyingsheet comprising a support havingV low thermal conductivity, and a heat-sensitivev layer containing, uniformly dispersed in a film-forming, transparent binder non-fusing at temperatures' below about 120 C., a multitude of closely spaced particles of at least two visibly inter-reactive solidsof which one is a liquid attemperatures above normal room and storage temperatures and with'- in the range of about 60-1-20" C. and is a waterinsoluble iron salt of an organic acidi, and the second particulate solid is a phenol, said interreactive solids in said heat-sensitive layer being stable at temperatures less than about 60C. and being rapidly visibly inter-reacted when the copying-sheet is heatedk to 120 C.

3. A heat-sensitive copying-sheet for making direct, high contrast, clear detail copies of graphic subject-matter asherein described, said copying-sheet comprising a support having low thermal conductivity, and a heat-sensitive layercomprising a nlm-forming, transparent binder non-fusing at temperatures below about 120 C., an iron salt of an organic acid, and a phenol' which is visibly reactive therewith; said iron salt being a solid at normalv room and storage temperatures and melting to a liquid'within therangeof about 60-120Av C., and said phenol being a solid at normal room and storage temperatures;` said iron salt being in the form rof particlesV distributed substantially uniformly throughout said heat-sensitive layer; said iron salt andi said` phenol in said heat-sensitive layer being stable at temperatures less thanV about 60 C. and being rapidly visibly inter-reacted whenthe copyingsheetI is heated to 120' C.

4. A heat-sensitive'y copying-sheet for making direct, high contrast, clear detail copies of graphic subject-matter as hereinA described', said; copyingsheet being non-sticking at 120 C. and comprising a support having low thermal conductivity, and a protected heat-sensitive. layer comprising an iron salt of an organic acid and a phenol visiblyl reactivev therewith, said iron salt being a solidv at' normal room and storage temperatures and melting to a liquid within the range'Y of about (iO-120 C., said iron salt and said phenol being firmly' bonded to said support in uniform mutually closely adjacent relationship over the entire active area thereof, said iron salt and said phenolv insaid heatsensitive layer being stable at temperatures less than about C. and being rapidly visibly inter-reactive when said copying-sheetis heated to C.

5. A heat-sensitive copying-sheet for making direct, high contrast, clear' detail copies` of graphic subject-matter as herein described, said copyingsheet' being non-sticking atL 120 C. and comprising a support having low thermal conductivity, and a4 protected heat-sensitive. layer comprising an iron salt of" an. organic acid and a phenol visibly reactive therewith, said iron salt and said phenol in said heat-sensitive. layer being firmly bonded to said support'v in mutually closely adjacent. relationship, being stable at temperature lessi than. aboutL 60 C., andbeing uniformly rapid- 1y visibly inter-reactive when said copying-sheet is heated to` 120 C., both the iron salt and the phenol being solid at. normali room temperatures and. at least one of said iron salt and phenol melting to. a liquid at a temperature within the range of about GII-120 C.

6. A heat-sensitive copying-sheetv for making direct, high contra-st, clear detail copies of graphic subject-matter as herein described, said copyingsheet: comprising; a, support. having low thermal conductivity, andv a heat-sensitive layer containing, uniformly dispersed in a-lm-forming, transparent binder non-fusing at,y temperatures below about 120 C., a multitude of closely spaced solid particles ofi materials comprising an iron salt of an organic acid and a phenol, at least one of said n'iaterials melting at atemperature within the range of about 60-120-c C. said materials being capable of rapidly visibly reacting at normalroom temperature, and said. materials in said heatsensitive layer being. stable at temperatures less than about 60 C. and being rapidly visibly interreacted whenV the; copying-sheet is heated to 120 C.

'1. 'The copying-sheet of claim 6, in which the support member is paper.y

CARL` S. MILLER. BRYCE L. CLARK.

No references cited'. 

1. A HEAT-SENSITIVE COPYING-SHEET FOR MAKING DIRECT HIGH CONTRAST, CLEAR DETAIL COPIES OF GRAPHIC SUBJECT-MATTER AS HEREIN DESCRIBED, SAID COPYINGSHEET COMPRISING A SUPPORT HAVING LOW THERMAL CONDUCTIVITY, AND A HEAT-SENSITIVE LAYER CONTAINING, UNIFORMLY DISPERSED IN A FILM-FORMING, TRANSPARENT BINDER NON-FUSING AT TEMPERATURES BELOW ABOUT 120* C., A MULTITUDE OF CLOSELY SPACED PARTICLES OF AT LEAST TWO VIBISLY INTER-REACTIVE SOLIDS OF WHICH ONE IS A LIQUID AT TEMPERATURES ABOVE NORMAL ROOM AND STORAGE TEMPERATURES AND WITHIN THE RANGE OF ABOUT 60-120 C. AND IS AN IRON SALT OF A SOLID LONG-CHAIN FATTY ACID, AND THE SECOND INTER-REACTIVE PARTICULATE SOLID IS A PHENOL SAID INTER-REACTIVE SOLIDS IN SAID HEAT-SENSITIVE LAYER BEING STABLE AT TEMPERATURES LESS THAN ABOUT 60C. AND BEING RAPIDLY VISIBLY INTER-REACTED WHEN THE COPYING SHEET IS HEATED TO 120* C. 